The invention relates to a sealed electrochemical cell comprising a negative electrode whose electrochemically active material includes an intermetallic compound which can form a hydride with hydrogen, the cell comprising an electrolyte of a solution of at least one alkali hydroxide in water, and the negative electrode having a discharge buffer.
The invention also relates to a method of manufacturing such a sealed electrochemical cell.
A rechargeable cell of this type is described in European Patent Application No. EP 170519. In this cell the negative electrode has a larger electrochemical capacity than the positive electrode to preclude the formation of gaseous hydrogen in the case of overcharging. In the case of overdischarging, which may occur, for example, when such cells are connected in series in a battery, oxygen evolution and corrosion may cause damage to the negative electrode. To preclude this a part of the excess capacity of the negative electrode is filled with a precharge or discharge buffer. The purpose of these measures is to increase the operating life of the electrochemical cell, i.e., to increase the number of possible charging and discharging cycles. Moreover, overdischarging would lead to a substantial pressure increase in the cell. If this causes a safety valve to open, electrolyte escapes from the cell and, hence, the capacity of the cell decreases.
Because it is difficult to safely precharge cells on a large scale by supplying hydrogen gas to the cells before they are sealed, a precharging element is used in the above-mentioned European Patent Application. This precharging element comprises a base metal M, for example a transition metal or an element of one of the groups III and IV of the Periodic Table of Elements, and is electrically conductively connected to the negative electrode. This negative electrode is partly charged by oxidation of the precharging element according to the following reaction: EQU M + n OH.sup.- .fwdarw. M(OH).sub.n + n.e.sup.-
and by electron transfer to the negative electrode.
This known method has the disadvantage that foreign ions arc present in the electrolyte. Moreover, to provide the cell with a precharging element which is connected to the negative electrode the construction of the cell must be adapted.